I had Gracie for 2 years by herself and I recently got Lily and they do not play together at all. Gracie loves playing with my parents minature sheltie but refuses to even have anything to do with Lily.
I would say dont just get another dog for company for your other dog...unless you are ready to raise another puppy
I was tempted to get another yorkie about 3 months after Gracie and I knew it would be too much dog for me at the time...
I think when you have one yorkie they are your best friend...once you get 2 or more they grow close to each other more....IMO